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Cycles, dipoles, and oscillations

NCAR scientists probe atmospheric patterns

February 2, 2011 | It's been a noticeably dry and gusty fall and winter thus far along the Front Range, with wildfires, wind storms, and wave clouds filling the skies. Meanwhile, early-season snowpack in Colorado’s mountains has measured well above average in many locations. The likely cause of this weather? La Niña, El Niño’s sister.

La Niña occurs when cooler-than-normal sea surface temperatures form in the eastern and central Pacific Ocean off the coasts of Peru and Ecuador. The cooler water temperatures are caused by an increase in easterly sea surface winds that force cold water from below the ocean’s surface to the top. The cooler water, in turn, chills the overlying air and helps reinforce the La Niña pattern.

If what goes around comes around, as the saying goes, nowhere is this more evident than with atmospheric patterns such as La Niña. A handful of major weather and climate patterns with names that range from the fanciful (Pineapple Express) to the more technical (Pacific Decadal Oscillation) stretch thousands of miles across the atmosphere and shape weather and climate in disparate places. These circulation patterns arise because of heating contrasts between the poles and equator, modulated by the changing seasons, and the rates at which land and water absorb and release heat. The result is an ever-changing patchwork of warmer, cooler, wetter, and drier regions.

At NCAR, scientists in NESL/CGD and other divisions are working to better understand and predict these patterns, as they have considerable effects on regional temperature and precipitation. They’re also looking at the interplay between atmospheric patterns and climate change.

Children of the tropics

Better known for the El Niño and La Niña patterns it produces, ENSO (El Niño–Southern Oscillation) can cause extreme weather around the world, including floods and droughts. It is characterized by variations in sea surface temperatures in the eastern tropical Pacific, with the warm phase called El Niño and the cool phase La Niña, along with a seesaw in atmospheric pressure in the tropical western Pacific known as the Southern Oscillation.

South American fishermen gave El Niño its name (Spanish for “The Boy”) in reference to the Christ child, because the periodic warming of Pacific waters off Peru and Ecuador is often noticed around Christmastime. The neutral phase of ENSO, during which the atmosphere and ocean are neither unusually warm nor cold, is sometimes humorously referred to as La Nada ("The Nothing").

A look at Earth’s major atmospheric patterns in their regions of origin: ENSO (El Niño/ Southern Oscillation), the Pacific Decadal Oscillation (PDO), the North Atlantic Oscillation (NAO), the Arctic Oscillation/ Northern Annular Mode (AO/ NAM), the Southern Annular Mode (SAM), the Indian Ocean Dipole (IOD), and the Atlantic Multi-Decadal Oscillation (AMO). View an interactive version of this map with more information about each pattern.

Rather than being mirror images, El Niño and La Niña display significant differences in their spatial structures and seasonal evolution. New research in CGD has reproduced some of the key differences using climate models. The study, led by Yuko Okumura, has important implications for the prediction of ENSO and its global influences.

Yuko and Clara Deser analyzed two datasets of monthly sea surface temperatures spanning different periods: 1900–2008 and 1982–2008. They confirmed a robust asymmetry between El Niño and La Niña throughout the record, especially during strong ENSOs. Both phases typically begin in late spring or summer. Most El Niños terminate rapidly after peaking in December or January, but many La Niñas persist through the following spring and summer and re-intensify in winter, some even lasting through a third year. Modeling experiments suggest that this asymmetry can be explained by the different evolution of surface wind anomalies over the far western Pacific during El Niño and La Niña.

The surface wind anomalies associated with the current La Niña show a pattern consistent with Yuko and Clara’s analysis, as NOAA’s Climate Prediction Center reports that the present La Niña is expected to continue well into spring 2011 in the Northern Hemisphere.

The ability of climate models to capture ENSO got a boost in April 2010 with the release of CCSM4, which is being used for an ambitious set of climate experiments that will be featured in the IPCC’s next report, due in 2014.

CCSM4 depicts ENSO better than previous versions of the model, according to CGD’s Rich Neale. The model is more sensitive to tropospheric humidity, making it better able to represent deep convection in the tropics. It’s also more sensitive to vertical wind shear. In addition, the model reproduces the observed asymmetry in the durations of El Niño and La Niña, which was not captured by the earlier versions of CCSM.

The north Pacific

Related to ENSO is the Pacific Decadal Oscillation (PDO) in the north Pacific. During the PDO’s positive phase, sea surface temperatures tend to be above average along the west coast of North America and in the eastern tropical Pacific, while across the central north Pacific they are cooler than average. The opposite pattern occurs during the negative phase. Each phase typically persists for 10–30 years. A warm phase predominated from the late 1970s to around 2000, but the PDO has alternated between cold and warm phases since then.

In CGD, Haiyan Teng and Grant Branstator are using CCSM4 to study the predictability of the PDO, in support of decadal predictions for the next IPCC report. Even though the PDO has an oscillating period of 20–50 years, it may not be predictable for that long, Haiyan says. She and Grant are estimating the limit of PDO predictability by running large ensemble experiments in which they measure how quickly small errors spread from initial conditions in the model.

“Even if we have the fastest computers, we cannot predict weather after two weeks because the system is chaotic and a tiny error in the initial states will grow too big,” she explains. “The same thing happens to the climate system—we’re improving the models and have faster computers but we don’t know if even the range of 10-30 years is actually predictable.”

The two haven’t yet looked at the influence of climate change on the PDO but plan to address this in future research.

Polar patterns

Swirling around the northern half of the globe—and grabbing attention during recent bouts of cold and snow in the United States and Europe—are the North Atlantic Oscillation (NAO) and the Northern Annular Mode (NAM), the latter sometimes known as the Arctic Oscillation.

The NAO affects weather in Europe and along the east coast of North America. In its positive phase, pressure over the Arctic drops lower than normal, while pressure rises more than average near the Azores; in the negative phase, this pattern is reversed. The NAM is closely related to the NAO. Its positive phase is associated with a stronger and more northerly vortex encircling the pole and fewer intrusions of cold Arctic air into midlatitudes, whereas the negative phase brings a weaker, more variable vortex and greater risk of Arctic outbreaks of cold air into eastern North America and Europe.

Starting in the 1960s, the NAO/NAM trended toward more positive values before tapering off in the mid-1990s. The last two winters have seen extreme negative values. Climate models have predicted that the build-up of greenhouse gases in the atmosphere will cause the NAO/NAM to trend positive, but Clara cautions that the modes vary naturally.

A schematic of the major features of the positive (left) and negative (right) modes of the NAO/NAM. (Illustration by John Michael Wallace, University of Washington, courtesy NSIDC Education Center.)

“Increasing carbon dioxide in most of the models does show a weak positive response, but that signal is quite small compared to natural ups and downs,” she says. “It would be premature to say that a winter with a strong positive NAO/NAM would be entirely attributable to greenhouse gases.”

One question that Clara and other scientists are studying is how the projected loss of Arctic sea ice due to climate change will affect the NAO/NAM. Modeling evidence suggests that the disappearance of sea ice would favor a negative NAO/NAM, the opposite effect of greenhouse gas forcing. Despite the negative NAO/NAM conditions, however, temperatures over northern Eurasia warm due to enhanced heat loss from the adjacent Arctic Ocean.

“The competition between the strength of the atmospheric circulation response which favors colder temperatures and the extent of Arctic sea ice loss which favors warmer conditions makes this an interesting problem and an open area of research,” Clara says. (More on this topic will appear in the winter 2011 issue of UCAR Magazine.)

On the other side of the world is the Southern Annular Mode (SAM), also known as the Antarctic Oscillation and analogous to the NAM. In the SAM's positive phase, when sea level pressure is low over Antarctica and high in the midlatitude region to the north, a stronger and more southerly vortex encircles the pole, leading to fewer intrusions of Antarctic air into the southern oceans. The negative phase features a weaker, more variable vortex and a greater risk of Antarctic outbreaks of cold air heading north.

The SAM has trended toward positive values since the 1960s, a development that scientists attribute to the effects of both ozone depletion above Antarctica and increases of greenhouse gas emissions. Scientists expect the ozone hole to recover over the coming century, raising the question of how this will affect the SAM. Some projections have shown that greenhouse gas forcing will dominate, with the positive trends that began in the 20th century continuing into the future. Other studies have found the opposite response, with the dominance of ozone recovery overwhelming the influence of greenhouse gases and leading to a reversal of positive summertime trends.

A recent study led by Julie Arblaster (CGD) examines simulations from two coupled climate models that incorporate greenhouse gases and ozone recovery. While both models suggest that recent positive summertime SAM trends will reverse sign over the coming decades as the ozone hole recovers, climate sensitivity in the models appears to play a large role in modifying the strength of how the mode responds.

“Future changes in the SAM could also have important impacts on Southern Ocean carbon uptake,” Julie says. “Understanding the mechanisms behind the various model results is an important step toward narrowing the uncertainty in future climate projections.”

Everything you've ever wanted to know about atmospheric patterns

Among some audiences, "Pineapple Express" may be best known as the name of a 2008 comedy by director Judd Apatow. In scientific circles, however, the term is an informal name for the weather pattern that brings warm, moist air from the tropical Pacific Ocean near Hawaii to the Pacific Northwest and California, producing heavy rains. The Pineapple Express often forms when a dip in the jet stream coincides with atmospheric moisture associated with the Madden-Julian Oscillation.

Because the moisture may originate far from Hawaii, scientists are increasingly referring to the phenomenon as atmospheric rivers (ARs). James Done (NESL/MMM) recently collaborated with NOAA to simulate and visualize two AR cases as part of a project called ARkSTORM. Led by the U.S. Geological Survey, ARkSTORM examined past atmospheric rivers and associated flooding to produce a scenario that will help planners in California prepare for the next "big one."

Rainfall accumulating along the west slopes of California’s mountains appears in bright colors in this visualization of a winter storm produced by James Done (NESL/MMM), based on NOAA work using the Weather Research and Forecasting model. The simulation was in support of the ARkSTORM project.